1. The present invention relates to a drive arrangement for a four wheel drive vehicle having a steerable axle, in particular, for agricultural tractors.
2. Description of the Related Art
DE-OS No. 3 408 991 provides a two stage drive arrangement with a dual (input side/output side) planetary gear set. The main vehicle transmission drives the planet carrier of the input side planetary. The planet carrier of the output side planetary is connected to the ring gear of the input side planetary, while the ring gear of the output side planetary can be locked to a housing. The sun gears of both planetaries are rigidly connected to an output shaft which leads to the selectively driven wheels of a front axle. In the first transmission stage, the input side planetary is locked, that is, the planet carrier and sun gear of the input side planetary are locked against relative rotation, and direct drive is provided from the main transmission to the input shaft. In the second transmission stage, the ring gear of the output side planetary is locked against rotation, and a speed increase is obtained between the main transmission and input shaft. Shifting between stages is controlled by electromagnetic valves, which in turn are controlled by the steering angle of the wheels provided on the front axle. Automatic shifting to the second stage is initiated when the steering angle is such that the slip or overrun of the front wheels with respect to the rear wheels reaches zero, i.e., when the front wheels are describing a larger steering radius than the rear wheels. Alterntively, the second stage is initiated at low vehicle speeds, and whenever the front wheels have a lower circumference speed than the rear wheels, e.g., during braking.
This known drive arrangement has the disadvantage that after shifting from the first to the second stage, the front wheels have an excessively high increase in overrun, which affects the steering movement and damages the tires and the ground, especially grassy turf.
The problem underlying this invention is seen as providing a drive arrangement which avoids this excessive overrun.